Prong binders are loose leaf binders that have curved arcuate prongs which each extend from one cover to the other through the entire sheaf of pages.
These binders have the advantage of being able to hold a greater number of pages than ordinary ring binders without fraying the pages. They are able to hold almost as many pages as straight post binders, but have the advantage of being able, because of the curvature of the prongs, to be placed in a position where the pages can be easily read.
The difficulty with prong binders is that the mechanism to open the prongs is quite complex and unwieldy. One of the most common mechanisms is a hinge which is located along the center of the backbone with the backbone folding to open the prongs. Such a mechanism is shown in U.S. Pat. No. 1,812,435. The problem with this type of mechanism is that it is not pleasing esthetically, requires difficult locking mechanism and it is very difficult to place printing, such as the title of the book, on the backbone, because the hinge splits the backbone into two thin sections.
Thus, there has been a desire to produce a prong binder which has a solid backbone, which does not fold or split. One such binder that lacks a folding backbone is constructed so that at least one of the supports which hold the prongs, disconnects from one side of the binder in order for the prongs to be opened without folding the backbone. This mechanism is shown in U.S. Pat. No. 2,791,220 (McBee). The mechanism provides two longitudinal supports, each of which supports two of the four prongs and one of which is detachably connected to a cover of the binder. In order to open and close the binder, the mechanism uses two hinges which connect the longitudinal supports to open and close the prongs. The hinges are "V" shaped in construction, with a single rivet connecting their two sections. They rotate from a flat horizontal position against the backbone of the binder to an upright position. In a flat horizontal position, the mechanism is closed and locked. When one of the supports is detached from the side wall, the hinges are then swung underneath to the upright position and the binder is opened. The problem with the mechanism is that the hinges are structurally weak. When the hinges are in the horizontal position, the supports can be shifted longitudinally with respect to each other with the hinges offering very little resistance to such shifting. This shifting causes bending and distortion of the hinges and possible breaking of the hinge rivet, all of which results in improper operation or non-operation of the mechanism. Also, when a hinge is swung to the upright position so that the binder can be opened, the weight of the free support is placed on the hinge in a manner such that a slight shifting of the support would tend to bend the hinge or break the hinge rivet and cause misalignment or improper operation of the mechanism. The problem is that by having the flat construction of the hinges with a single rivet on an axis perpendicular to the flat surface of the hinges, the forces on the hinge during their operation tend to bend and twist the hinges and cause their misalignment. The situation is also aggravated as there is nothing other than the single rivets to hold the two longitudinal supports in proper alignment. Because of this, there was a tendancy as the binder was in use for the prong bars to be shifted longitudinally so that the prongs were no longer in proper alignment.
The desire, therefore, was to have a binder mechanism whereby the hinges would support and keep the two longitudinal bars in proper parallel alignment with the hinges constructed so that they resist bending and misalignment when rotated between the open and closed positions.